Prefix search implementation

Implement a solution to the issue described by google/leveldb#610

leveldb-api/src/main/java/org/iq80/leveldb/DB.java

@@ -56,6 +56,18 @@ byte[] get(byte[] key, ReadOptions options)
     @Override
     DBIterator iterator();
 
+    /**
+     * Return a iterator over contents of the database that with keys with {@code prefix}.
+     *
+     * @param options iterator read options
+     * @param prefix keys prefix
+     * @return new iterator over a view o the database.
+     */
+    default DBIterator prefixIterator(ReadOptions options, byte[] prefix)
+    {
+        throw new UnsupportedOperationException();
+    }
+
     /**
      * Return a iterator over the contents of the database.
      * The result of {@link DB#iterator(ReadOptions)} will automatically position

leveldb-api/src/main/java/org/iq80/leveldb/DBIterator.java

@@ -63,6 +63,9 @@ public interface DBIterator
      */
     void seekToLast();
 
+    /**
+     * Close resources associated to this iterator. Not closing it may lead to memory leak.
+      */
     @Override
     void close();
 }